1. Field of the Invention
The present invention relates to a signal transmission line and a circuit board, and more specifically, relates to a signal transmission line and a circuit board in which a high-frequency signal is transmitted.
2. Description of the Related Art
As existing signal transmission lines, for example, the microstrip line and the strip line that are described in Japanese Unexamined Patent Application Publication No. 2004-152963 are known. FIG. 8A is a cross-sectional structure diagram of the microstrip line 200a described in Japanese Unexamined Patent Application Publication No. 2004-152963, and FIG. 8B is a cross-sectional structure diagram of the strip line 200b described in Japanese Unexamined Patent Application Publication No. 2004-152963. In FIGS. 8A and 8B, the up-down direction is defined as a lamination direction.
The microstrip line 200a shown in FIG. 8A is composed of a main body 202, a signal line 204, and a ground conductor 206. The main body 202 is composed of, for example, a plurality of laminated insulating sheets formed from a flexible material. The signal line 204 is a wiring in which a high-frequency signal is transmitted, and extends in the direction perpendicular to the sheet surface of FIG. 8A. A ground potential is applied to the ground conductor 206, and the ground conductor 206 is provided on the lower side of the signal line 204 in the lamination direction. The signal line 204 and the ground conductor 206 overlap each other when seen in a planar view from the lamination direction.
The microstrip line 200a described above is used, for example, for connecting a signal generation source to a load circuit in a state where impedance matching is provided therebetween. Specifically, in the microstrip line 200a, a capacitance occurs between the signal line 204 and the ground conductor 206, and the impedance of the signal line 204 decreases. Thus, impedance matching is provided among the signal generation source, the signal line 204, and the load circuit.
However, in the microstrip line 200a, unwanted emission occurs. Specifically, a high-frequency signal has a wavelength shorter than the electrical length of the microstrip line 200a. Thus, when a high-frequency signal is transmitted in the signal line 204, a plurality of standing waves is present in the signal line 204. Due to the standing waves, noise is emitted from the signal line 204. Of the noise, the noise emitted to the lower side in the lamination direction is absorbed by the ground conductor 206. However, the noise emitted to the upper side in the lamination direction is emitted from the signal line 204 to the outside of the main body 202.
Meanwhile, in the strip line 200b shown in FIG. 8B, unwanted emission can be suppressed. Specifically, the strip line 200b has a structure in which a ground conductor 208 is provided in the microstrip line 200a. As shown in FIG. 8B, the ground conductor 208 is provided on the upper side of the signal line 204 in the lamination direction. The signal line 204 and the ground conductor 208 overlap each other when seen in a planar view from the lamination direction. In the strip line 200b having the structure described above, noise emitted from the signal line 204 is absorbed by the ground conductor 208. Thus, in the strip line 200b, unwanted emission can be suppressed.
However, the strip line 200b has a problem that it is difficult to bend the strip line 200b in a U shape. Specifically, the strip line 200b is composed of three conductor layers, namely, the signal line 204 and the ground conductors 206 and 208. The signal line 204 and the ground conductors 206 and 208 are in general produced from a metal film such as copper foil, and thus it is difficult to bend these elements as compared to the insulating sheets constituting the main body 202. Therefore, the strip line 200b has a problem in that it is relatively difficult to bend the strip lines 200b in a U shape.
As described above, in the existing microstrip line 200a and strip line 200b, it is difficult to achieve both suppression of unwanted emission and easy bending in a U shape.